Cathode active material comprising additive for improving overdischarge-performance and lithium secondary battery using the same

ABSTRACT

Disclosed is a cathode active material providing a cell performance that is not adversely affected by overdischarge, and a lithium secondary cell using the same. More particularly, the cathode active material for a lithium secondary cell comprises a lithium-transition metal oxide capable of lithium ion intercalation/deintercalation, wherein the cathode active material further comprises a lithium manganese oxide having a layered structure represented by the following formula 1 as an additive:[formula 1] LiM x Mn 1-x 0 2  wherein, x is a number satisfying 0.05 x&lt;0.5, and M is at least one metal selected from the group consisting of Cr, Al, Ni, Mn and Co. The lithium manganese oxide of formula 1 used as an additive for a cathode active material of a lithium secondary cell provides lithium ions in such an amount as to compensate for an irreversible lithium ion-consuming reaction at an anode, or more, thereby providing a lithium secondary cell which is low in capacity loss by over-discharge.

TECHNICAL FIELD

The present invention relates to a lithium secondary cell, the capacityof which is not significantly reduced after over-discharge and thecapacity restorability of which after over-discharge is excellent, andmore particularly, to a cathode active material comprising a lithiummanganese oxide (LiM_(x)Mn_(1-x)O₂) having a layered structure as acathode additive for improving over-discharge property, and a lithiumsecondary cell obtained by using the same.

BACKGROUND ART

Recently, as mobile communication industries and information electronicindustries progress in various technologies, a light-weight lithiumsecondary cell having a high capacity is increasingly in demand.However, a lithium secondary cell may ignite and explode due to extremeheat emission, when it is over-charged or is in a short circuit state.Moreover, when a lithium secondary cell is over-discharged below anormal voltage range, its capacity is rapidly reduced so that it may notbe used any more.

For these reasons, a safety device like a protection circuit, a PTC,etc., has been attached to a lithium secondary cell since lithiumsecondary cells were first developed. However, such protection circuits,PTCs, etc., are not preferable, because they are expensive and take up alarge volume, thereby increasing the price, volume and weight of thecell. Therefore, developments for a cell having a reduced manufacturingcost and an increased cell capacity without using such a protectioncircuit, PTC, etc., are very much in demand.

Conventionally, an organic or an inorganic additive is used in anon-aqueous electrolyte solution, or the outer structure of a cell ischanged for the purpose of ensuring the cell safety when a cell isover-charged or is in a short circuit state. However, in the case that acell is over-discharged below an adequate voltage, even if one tries tocharge the cell again, the cell capacity is so rapidly reduced thatcharge/discharge of the cell may not be accomplished any more.

Conventional lithium secondary cells developed up to date have astructure in which discharge is limited and terminated by an anode inthe case of over-discharge. Particularly, when a non-aqueous lithiumsecondary cell is first charged, a solid electrolyte interface (SEI)film is formed on the surface of an anode. In this case, a great amountof lithium ions released from a cathode are used, and thus the amount ofLi participating in charge/discharge is reduced. When over-dischargingoccurs in the state in which the amount of Li is reduced, activated Lisites in the cathode are not fully occupied and the cathode voltage isnot decreased below a certain voltage. Therefore, discharge isterminated by the anode (see FIG. 7).

Meanwhile, a cell capacity is rapidly reduced by the following reasons.A cell voltage is defined by a difference of a cathode voltage and ananode voltage. Additionally, when a cell is continuously discharged at alow electric current, even after the cell voltage is decreased below ageneral-use voltage, the cathode voltage is not decreased any more dueto the consumption of Li in the anode, and thus it is slowly decreased.On the other hand, the anode voltage is rapidly increased and eventuallyit is raised to 3.6 V, at which point a copper foil used as an anodecollector is oxidized. Thus, the copper foil is dissolved in a copperion state, contaminating electrolytes, is attached again to the surfaceof the anode during re-charge, and thus the anode active materialbecomes unusable. Therefore, when the oxidization of the copper foiloccurs, the cell capacity is rapidly reduced after over-discharge, sothat the cell becomes unusable.

Accordingly, it is desirable to develop a cell in which the celldischarge is limited by a cathode, so that the cell capacity may not besignificantly reduced after over-discharge, and a new method for makingsuch a cathode-limited cell is very much in demand.

Meanwhile, when a lithium manganese oxide is used as a cathode activematerial, a spinel-structured lithium manganese oxide is generally usedfor the purpose of improving the thermal stability of a cathode. Thisprovides an advantage of a low cost and a simple synthetic procedure.However, the cell using a spinel-structured lithium manganese oxide as acathode active material has problems that the capacity is low, the celllife may be reduced by side reactions, the high-temperature property ispoor and the conductivity is also low. In order to solve these problems,many attempts to use a spinel-structured lithium manganese oxidepartially substituted with other metals have been made. KoreanUnexamined Patent Publication No. 2002-65191 discloses aspinel-structured lithium manganese oxide having excellent thermalstability, however, it provides a low capacity and cannot improve theover-discharge preventing capability.

In order to solve the problem of the low capacity in the spinel and toensure excellent thermal stability of a manganese-based active material,many attempts to use a lithium manganese oxide having a layeredstructure have been made. In this case, the layered structure isunstable, and thus a phase transition occurs during charge/discharge,the cell capacity is rapidly reduced and the cell life is decreased. Tosolve these problems, methods for maintaining the structural stabilityby doping or substituting with other metals have been suggested.Particularly, Korean Unexamined Patent Publication No. 2002-24520discloses a cell, in which a lithium manganese oxide having a layeredstructure is used as a cathode active material having excellent thermalstability, and a phase transition is prevented during charge/dischargeso that the cell life can be improved. However, the over-dischargepreventing capability cannot be improved in this case.

DISCLOSURE OF THE INVENTION

The present inventors tried to develop a cell, in which by using alithium manganese oxide having a layered structure, the cell dischargeis limited by a cathode, so that the cell capacity may not besignificantly reduced after over-discharge.

We found that, when a lithium manganese oxide having a layered structureis used as an additive for a cathode active material, a phase transitionfrom a layered structure to a spinel structure in the lithium manganeseoxide controls irreversible reactions in a cathode and an anode, andthus the cell capacity is not significantly reduced afterover-discharge.

Therefore, the present invention has been made based on the foregoing,and it is an object of the present invention to provide a cathode activematerial for a lithium secondary cell comprising a lithium manganeseoxide having a layered structure as an additive for a cathode, and alithium secondary cell obtained by using the same.

According to an aspect of the present invention, there is provided acathode active material for a lithium secondary cell comprising alithium-transition metal oxide capable of lithium ionintercalation/deintercalation, characterized by further comprising alithium manganese oxide having a layered structure represented by thefollowing formula 1 as an additive:LiM_(x)Mn_(1-x)O₂  [formula 1]wherein, x is a number satisfying 0.05≦x<0.5, and M is at least onemetal selected from the group consisting of Cr, Al, Ni, Mn and Co.

There is also provided a lithium secondary cell obtained by using thesaid cathode active material.

The lithium secondary cell according to the present invention comprises:(a) a cathode comprising the said cathode active material according tothe present invention, (b) an anode, (c) a separator, and (d) anon-aqueous electrolyte solution containing a lithium salt and anelectrolyte compound.

The present invention will be explained in detail hereinafter.

The lithium manganese oxide used as an additive for a cathode activematerial according to the present invention is represented by thefollowing formula 1 and has a layered structure:LiM_(x)Mn_(1-x)O₂  [formula 1].wherein, x is a number satisfying 0.05≦x<0.5, and M is at least onemetal selected from the group consisting of Cr, Al, Ni, Mn and Co.

The lithium manganese oxide of formula 1 (LiM_(x)Mn_(1-x)O₂) has alayered monoclinic, orthorhombic or hexagonal structure, and can beprepared by mixing lithium carbonate (Li₂CO₃), manganese oxide (Mn₂O₃)and a metal oxide in solid phases and heat-treating the mixture at ahigh temperature under argon atmosphere.

The lithium manganese oxide of formula 1 can act as a cathode activematerial, in which a structural change into a spinel structurerepresented by the following formula 2 occurs, when a cell ischarged/discharged first:LiM_(2x)Mn_(2-2x)O₄  [formula 2]wherein, x is a number satisfying 0.05≦x<0.5, and M is at least onemetal selected from the group consisting of Cr, Al, Ni, Mn and Co.

The lithium manganese oxide of formula P having a layered structure isshown in FIG. 1, and the lithium manganese oxide of formula 2 having aspinel structure is shown in FIG. 2.

The lithium manganese oxide of formula 1 having a layered structuredeintercalates one mole of lithium per two oxygen atoms during the firstcharge, however, after the first charge/discharge cycle, due to thestructural change into a spinel structure, it becomes a substancecapable of lithium intercalation/deintercalation in the ratio of 0.5mole of lithium per two oxygen atoms.

Accordingly, when the lithium manganese oxide of formula 1 having alayered structure is used in a cathode as an additive for a cathodeactive material, the cathode active material composition according tothe present invention shows a large difference between initial chargecapacity and initial discharge capacity. This irreversible capacityprovides lithium ions in such an amount as to compensate for anirreversible lithium consumption reaction in an anode caused by the SEIfilm formation on the surface of the anode during the first charge, ormore. Therefore, such amount of lithium ions may compensate for the highand irreversible capacity of the anode at the first charge/dischargecycle.

In addition, the cathode active material composition according to thepresent invention, which comprises a lithium-transition metal oxidecapable of lithium ion intercalation/deintercalation and the lithiummanganese oxide of formula 1 having a layered structure can inhibit thecapacity reduction caused by over-discharge, due to the irreversibilityof the lithium manganese oxide of formula 1 during the firstcharge/discharge cycle. This mechanism is shown in FIG. 7.

A cell voltage is defined by the difference of electric potentialsbetween a cathode and an anode. Over-discharge of a cell continuouslyproceeds until the cell voltage becomes 0 V, at which point the electricpotentials of a cathode and an anode are the same.

As mentioned above, in general, the voltage of an anode having arelatively high irreversible capacity increases rapidly, when anover-discharge occurs, and thus copper ions are dissolved from an anodecollector, so that charge/discharge cycles may not progresssuccessfully. In order to prevent the increase of the voltage in theanode during an over-discharge, it is desirable to increase theirreversible capacity of the cathode so as to decrease the voltage ofthe cathode rapidly. For the purpose of increasing the irreversiblecapacity of the cathode, the present invention adopted a method that anadditive having a high irreversible capacity is added to a cathode.

In the above formula 1, x is a number satisfying 0.05≦x<0.5, preferably0.05≦x<0.2. If x is less than 0.05, a side reaction such as manganeseion dissolution may be generated, while if x is 0.5 or more, a phasetransition from a layered structure to a spinel structure does not occurin a charge/discharge cycle, and thus it is not possible to improve theover-discharge property.

In the above formula 1, M is selected from the group consisting of Cr,Al, Ni, Mn and Co, and functions as a structure stabilizer. Preferably,M is Cr or Al. If M is Cr or Al, the structure of formula 1 is morestabilized, and provides excellent high-temperature life andhigh-temperature shelf property.

Most preferably, the lithium manganese oxide of formula 1 isLiCr_(0.1)Mn_(0.9)O₂.

The lithium manganese oxide of formula 1 (LiM_(x)Mn_(1-x)O₂) ispreferably added in an amount of 1 to 50 parts by weight based on 100parts by weight of a transition metal oxide. When the content of thelithium manganese oxide of formula 1 is less than 1 part by weight, itis not possible to solve the problem in the anode, such as copper iondissolution. Additionally, when the said content is more than 50 partsby weight, the voltage of the cathode is decreased rapidly during anover-discharge test, so that reduction of an electrolyte may occur inthe surface of the cathode and the cell capacity may be decreased.Therefore, in order to solve both problems in the cathode and the anode,the cathode potential preferably ranges from 2 V to 3.6 V and the anodepotential preferably 3.6 V or less, when the full cell voltage becomes 0V.

As described above, when the compound of formula 1 according to thepresent invention, preferably LiCr_(0.1)Mn_(0.9)O₂, is added to acathode of a cell comprising an anode active material having anirreversible capacity of 30% or less, as an additive for a cathodeactive material, it is possible to obtain a capacity restorability of90% or more after an over-discharge test and to prevent the decrease ofthe cell capacity. When the irreversible capacity of the anode activematerial is more than 30%, the cell capacity is reduced, and thus thecompound of formula 1 must be added to the cathode in an amount of 50 wt% or more of the cathode active material. Such an excessive addition ofthe compound of formula 1 may cause other problematic side reactions,the deterioration of life characteristics and cell capacity reduction.

In addition, according to the present invention, if the compound offormula 1 is added to the cathode to the extent of compensating for theirreversible capacity of the anode, it is possible to obtain veryexcellent performance in an over-discharge test of a SCF (safety circuitfree) cell, which does not need a protection circuit and is of interestto cell production companies recently.

The cathode active material used in the present invention is any one ofgeneral cathode active materials, however, it is preferable to use alithium-transition metal oxide. For example, at least onelithium-transition metal oxide selected from the group consisting ofLiCoO₂, LiNiO₂, LiMnO₂, LiMn₂O₄, Li(Ni_(a)Co_(b)Mn_(c))O₂(0<a<1, 0<b<1,0<c<1, a+b+c=1), LiNi_(1-d)CO_(d)O₂, LiCo_(1-d)Mn_(d)O₂,LiNi_(1-d)Mn_(d)O₂ (0≦d<1), Li(Ni_(x)Co_(y)Mn_(z))O₄ (0<x<2, 0<y<2,0<z<2, x+y+z=2), LiMn_(2-n)Ni_(n)O₄, LiMn_(2-n)Co_(n)O₄ (0<n<2),LiCoPO₄, LiFePO₄, etc., may be used, and preferably, LiCoO₂ is used.

As an anode active material, graphite, carbon, lithium metal and alloy,etc., that are capable of lithium ion intercalation/deintercalation, maybe used. Preferably, artificial graphite is used. The anode may comprisea binder, in which the binder is preferably PVDF (Polyvinylidenefluoride) or SBR (Styrene Butadiene Rubber).

As a separator, a porous separator is preferably used. For example, apolypropylene-, a polyethylene- or a polyolefin-based porous separatormay be used, but it is not limited thereto.

The electrolyte solution used in the present invention is a non-aqueouselectrolyte solution and may comprise a cyclic carbonate and a linearcarbonate. The cyclic carbonate includes, for example, ethylenecarbonate (EC), propylene carbonate (PC) and gamma-butyrolactone (GBL).Preferably, the linear carbonate includes, for example, at least onecarbonate selected from the group consisting of diethyl carbonate (DEC),dimethyl carbonate (DMC), ethylmethyl carbonate (EMC) and methylpropylcarbonate (MPC).

Additionally, the electrolyte solution used in the present inventioncomprises a lithium salt in addition to the said carbonate compound.Particularly, the lithium salt is preferably selected from the groupconsisting of LiClO₄, LiCF₃SO₃, LiPF₆, LiBF₄, LiAsF₆ and LiN(CF₃SO₂)₂.The lithium secondary cell according to the present invention ismanufactured by a conventional method, i.e., by inserting a porousseparator between a cathode and an anode and introducing an electrolytesolution.

Preferably, the lithium secondary cell according to the pre setinvention has the shape of a cylindrical can, an angular cell or apouch.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a structural model of a layered structure of the additive fora cathode active material represented by formula 1, before charge.

FIG. 2 is a structural model of a spinel structure of the additive for acathode active material represented by formula 2, after initialcharge/discharge.

FIG. 3 is a graph showing the result of a structural analysis of theadditive for a cathode active material represented by formula 1, byX-ray diffraction.

FIG. 4 is a graph showing the result of a structural analysis by X-raydiffraction, before and after a charge/discharge test of a coin typecell, when the lithium manganese oxide of formula 1 having a layeredstructure was used as an additive for a cathode active material.

FIG. 5 is a curve showing the current and the cell voltage according toa charge/discharge test of the cell using the additive for a cathodeactive material according to the present invention.

FIG. 6 is a graph showing the cell capacity test results of initial 50charge/discharge cycles, when the lithium manganese oxide having alayered structure represented by formula 1 is used as an additive for acathode active material in a coin-type cell.

FIG. 7 is a graph showing the cathode potential and the anode potential,before and after using the additive for a cathode active materialaccording to the present invention.

FIG. 8 is a diagram showing the over-discharge test results of thefollowing Example 1 and Comparative Example 1.

FIG. 9 is a graph showing a full cell voltage during the over-dischargetest of Comparative Example 1.

FIG. 10 is a graph showing a full cell voltage during the over-dischargetest of Example 1.

BEST MODE FOR CARRYING OUT THE INVENTION

Reference will now be made in detail to the preferred embodiments of thepresent invention. It is to be understood that the following examplesare illustrative only and the present invention is not limited thereto.

Example 1

A pouch-type polymer cell of 383562 size was manufactured by aconventional method.

LiCoO₂ was used as a cathode active material and LiCr_(0.1)Mn_(0.9)O₂was added in the amount of 8 parts by weight based on 100 parts byweight of the cathode active material.

LiCr_(0.1)Mn_(0.9)O₂ was prepared by mixing lithium carbonate, manganeseoxide and chrome oxide in solid phases, heat-treating the mixture at atemperature of 1000° C. under argon atmosphere for 12 hours, pulverizingthe heat-treated mixture and further heat-treating the pulverizedmixture at a temperature of 1100° C. under argon atmosphere for 12hours.

Additionally, Super-p and PVDF polymer, used as a conductive agent and abinder, respectively, were added to NMP as a solvent to form cathodemixture slurry, and then the slurry was coated on an Al collector toobtain a cathode. On the other hand, artificial graphite and copper wereused as an anode active material and an anode collector, respectively,and an EC/PC/DEC-based electrolyte solution containing 1M LiPF₆ was usedto obtain a cell by a conventional method.

Comparative Example 1

Example 1 was repeated to obtain a cell, except that the additive for acathode active material (LiCr_(0.1)Mn_(0.9)O₂) was not used in thecathode.

Experimental Example 1

FIG. 3 is a graph showing the result of a structural analysis of thelithium manganese oxide, LiCr_(0.1)Mn_(0.9)O₂, used as an additive for acathode active material in Example 1 by X-ray diffraction. According toFIG. 3, it is apparent that the lithium manganese oxide of formula 1 isa compound having a layered structure.

On the other hand, as shown in FIG. 4, the lithium manganese oxidehaving a layered structure, LiCr_(0.1)Mn_(0.9)O₂, was structurallychanged into a spinel structure, after a coin-type cell obtained byusing the same compound as an additive for a cathode active materialexperienced initial charge/discharge.

Additionally, as demonstrated in FIG. 5 showing the firstcharge/discharge capacity of a coin-type cell obtained by using thelithium manganese oxide of formula 1 having a layered structure as anadditive for a cathode active material, the cell provided a very lowfirst charge/discharge efficiency. As demonstrated in FIG. 6 showing thecharge capacity and the discharge capacity in the initial 50charge/discharge cycles, the lithium manganese oxide provided a very lowfirst charge/discharge efficiency. However, a charge/dischargeefficiency of about 100% could be obtained in the followingcharge/discharge cycles, and thus reversible lithiumintercalation/deintercalation could occur.

Experimental Example 2

A charge capacity and a discharge capacity before and after anover-discharge test were determined using each of the pouch-type polymercells of 383562 size obtained from Example 1 and Comparative Example 1,through a conventional method. The over-discharge test results are shownin FIG. 8. Each of the numbers means a discharge capacity restorabilityat 0.2C and 1C after over-discharge, based on a discharge capacity at0.2 C and 1 C before over-discharge. As shown in FIG. 8, Example 1according to the present invention provided a discharge capacityrestorability of 90% or more after an over-discharge test, and thusprovided an excellent over-discharge preventing effect compared toComparative Example 1.

Experimental Example 3

In order to demonstrate the effect of the additive for a cathode activematerial on over-discharge, a three-electrode experiment was performedusing the cells of Example 1 and Comparative Example 1. A base electrode(reference electrode) made of lithium metal was inserted to each of thepouch-type polymer cells of 383562 size obtained from Example 1 andComparative Example 1. Then, the potential differences between thereference electrode and each of the cathode and the anode were measuredin order to check how the cathode potential based on the base electrodeand the anode potential based on the base electrode were changed in apractical cell during charge/discharge cycles.

In the case of Comparative Example 1, the anode voltage increased duringan over-discharge test and a plateau in which copper ions were dissolvedout was present, as can be seen from FIG. 9. On the other hand, in thecase of Example 1, a plateau in which copper ions were dissolved was notpresent, as can be seen from FIG. 10.

Therefore, according to the present invention, LiCr_(0.1)Mn_(0.9)O₂providing a large irreversible capacity at the first charge/dischargecycle is added in order to control the irreversible capacities of thecathode and the anode adequately, and thus it is possible to prevent theincrease of the anode voltage in an over-discharge test so that the cellcapacity may not be significantly reduced after the over-discharge test.

INDUSTRIAL APPLICABILITY

As can be seen from the foregoing, according to the present invention,the compound of formula 1, preferably LiCr_(0.1)Mn_(0.9)O₂, is added toa cathode as an additive for a cathode active material to improveover-discharge properties, and the additive for a cathode activematerial can provide lithium ions in such an amount as to compensate forthe irreversible capacity of an anode, or more. Accordingly, the anodevoltage can be prevented from increasing during an over-discharge testso that a cell capacity restorability of 90% or more may be obtainedafter the over-discharge test.

While this invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not limited to thedisclosed embodiment and the drawings, but, on the contrary, it isintended to cover various modifications and variations within the spiritand scope of the appended claims.

1. A cathode active material for a lithium secondary cell comprising alithium-transition metal oxide capable of lithium ionintercalation/deintercalation, characterized by further comprising alithium manganese oxide having a layered structure represented by thefollowing formula 1 as an additive:LiM_(x)Mn_(1-x)O₂  [formula 1] wherein, x is a number satisfying0.05≦x<0.5, and M is at least one metal selected from the groupconsisting of Cr, Al, Ni, Mn and Co.
 2. The cathode active materialaccording to claim 1, wherein the content of the lithium manganese oxidehaving a layered structure is 1 to 50 parts by weight based on 100 partsby weight of the lithium-transition metal oxide.
 3. The cathode activematerial according to claim 1, wherein the lithium manganese oxidehaving a layered structure is LiCr_(0.1)Mn_(0.9)O₂.
 4. The cathodeactive material according to claim 1, wherein the lithium manganeseoxide is at lest one material selected from the group consisting of:LiCoO₂, LiNiO₂, LiMnO₂, LiMn₂O₄, Li(Ni_(a)Co_(b)Mn_(c))O₂,LiNi_(1-d)Co_(d)O₂, LiCo_(1-d)Mn_(d)O₂, LiNi_(1-d)Mn_(d)O₂,Li(Ni_(x)Co_(y)Mn_(z))O₄, LiMn_(2-n)Ni_(n)O₄, LiMn_(2-n)Co_(n)O₄,LiCoPO₄ and LiFePO₄, wherein 0<a<1, 0<b<1, 0<c<1, a+b+c=1, 0≦d<1, 0<x<2,0<y<2, 0<z<2, x+y+z=2, and 0<n<2.
 5. A lithium secondary cell comprisinga cathode, an anode, a separator, and a non-aqueous electrolyte solutioncontaining a lithium salt and an electrolyte compound, wherein thecathode comprises a cathode active material comprising alithium-transition metal oxide capable of lithium ionintercalation/deintercalation and a lithium manganese oxide having, alayered structure represented by the following formula 1 as an additive:LiM_(x)Mn_(1-x)O₂  [formula 1] wherein, x is a number satisfying0.05≦x<0.5, and M is at least one metal selected from the groupconsisting of Cr, Al, Ni, Mn and Co.
 6. The lithium secondary cellaccording to claim 5, wherein the lithium manganese oxide having alayered structure represented by the following formula 1, which iscontained in the cathode active material, is changed into a lithiummanganese oxide having a spinel structure represented by the followingformula 2 by the first charge/discharge cycle of the lithium secondarycell:LiM_(x)Mn_(1-x)O₂  [formula 1]LiM_(2x)Mn_(2-2x)O₄  [formula 2] wherein, x is a number satisfying0.05≦x<0.5, and M is at least one metal selected from the groupconsisting of Cr, Al, Ni, Mn and Co.
 7. The lithium secondary cellaccording to claim 5, wherein the lithium salt is at least one selectedfrom the group consisting of LiClO₄, LiCF₃SO₃, LiPF₆, LiBF₄, LiAsF₆ andLiN(CF₃SO₂)₂, and the electrolyte compound is at least one carbonateselected from the group consisting of ethylene carbonate (EC), propylenecarbonate (PC), gamma-butyrolactone (GBL), diethyl carbonate (DEC),dimethyl carbonate (DMC), ethylmethyl carbonate (EMC) and methylpropylcarbonate (MPC).
 8. The lithium secondary cell according to claim 5,wherein the content of the lithium manganese oxide having a layeredstructure is 1 to 50 parts by weight based on 100 parts by weight of thelithium-transition metal oxide.
 9. The lithium secondary cell accordingto claim 5, wherein the lithium manganese oxide having a layeredstructure is LiCr_(0.1)Mn_(0.9)O₂.
 10. The lithium secondary cellaccording to claim 5, wherein the lithium manganese oxide is at lest onematerial selected from the group consisting of: LiCoO₂, LiNiO₂, LiMnO₂,LiMn₂O₄, Li(Ni_(a)CO_(b)Mn_(c))O₂, LiNi_(1-d)CO_(d)O₂,LiCo_(1-d)Mn_(d)O₂, LiNi_(1-d)Mn_(d)O₂, Li(Ni_(x)Co_(y)Mn_(z))O₄,LiMn_(2-n)Ni_(n)O₄, LiMn_(2-n)Co_(n)O₄, LiCoPO₄ and LiFePO₄, wherein0<a<1, 0<b<1, 0<c<1, a+b+c=1, 0<1, 0<x<2, 0<y<2, 0<z<2, x+y+z=2, and0<n<2.